The present invention relates to a system for degassing fluids and, more particularly, to a system for degassing drilling muds.
There are numerous industrial operations where it is not only desirable but expedient that dissolved and/or entrained gas be removed from a liquid or slurry. An important example of this exists in the oil well drilling industry where the drilling mud used in the drilling operation frequently becomes contaminated with natural gas, hydrogen sulfide or other gases encountered in various downhole formations. The presence of such gases in the drilling mud decreases its weight and viscosity, thus markedly diminishing its effectiveness in preventing blowouts. Furthermore, some of the gases encountered in drilling operations, in addition to being toxic, are quite corrosive to drill strings and attendant equipment. Since it is uneconomical to continuously supply a source of new mud, it is necessary that the mud be recycled.
Numerous prior art systems have been proposed as systems for degassing drilling muds. Most of these systems utilize a degassing tank, maintained under vacuum, and various baffles and other such structures disposed interiorly of the tank designed to produce thin, flowing films of the mud to ensure a large surface area of the mud. The vacuum in the tank serves not only to draw the mud into the tank, but also to facilitate release of the gas from the thin film.
Several problems are associated with the use of a vacuum degasification system such as described above. For one, the degassing vessel, of necessity, requires the provision of means to maintain the necessary vacuum conditions within the degasser vessel. This can be accomplished with the use of an auxiliary vacuum pump such as disclosed in U.S. Pat. No. 3,241,295 or by various Venturi type ejector systems such as shown in U.S. Pat. No. 3,616,599. Such systems, as will be readily understood, present operational problems in that the level of mud and/or vacuum in the degassing vessel must be accurately controlled. As described in the two prior art patents noted above, this control is achieved by the use of relatively complicated float valve mechanism employed in the degassing vessel which, for example, in conjunction with the vacuum system such as the Venturi type dual ejector apparatus shown in U.S. Pat. No. 3,616,599, serve to maintain control of the vacuum in the degassing vessel. Such control systems are inherently complicated, costly to manufacture and pose potentially substantial maintenance problems.
Other prior art mud degassing systems control the vacuum in the degassing vessel, and hence the mud level therein, by the simple expediency of a valve mechanism to admit air thereby reducing the vacuum and the mud inflow. The latter type systems suffer from the disadvantage that potentially explosive mixtures of admitted air and gas evolved from the return drilling mud may occur in the degassing vessel. The hazard to human life and property in the event of an explosion of a degassing system such as might occur on an offshore drilling rig is readily apparent.
U.S. Pat. No. 3,769,779 discloses a system for degassing fluids in which the degassing vessel is not operated under subatmospheric pressure conditions. In the system shown in the aforementioned patent, which is herein incorporated by reference, a centrifugal pump forces the gas laden drilling mud into a degassing vessel under pressure in such a manner that the drilling mud is deposited as a thin film on the inner surface of the wall of the degassing vessel whence it flows to the bottom of the vessel and through a suitable outlet for reuse in the drilling operations. Because such a large surface area of the drilling mud is achieved in the system described above, the release of the gas in the degassing vessel is highly efficient. Accordingly, the system does not require the use of an external vacuum source. Thus, the system may be considered to be an atmospheric degasser.
While the system described in aforementioned U.S. Pat. No. 3,769,779 is a highly efficient system for the degasification of drilling muds, since it does act as an atmospheric degasser, there is the possibility that air can be introduced into the degassing vessel through the outlet of the degasser thereby creating a potentially explosive gas mixture with the disengaged gas.